1. Field of the Invention
The present invention relates to an electric contact with a base metal for use in a current switch, such as an electromagnetic contactor.
2. Description of the Related Art
As is known, so-called non-oxide contact materials of Ag and Ag-Ni, or Ag-oxide contact materials in which an oxide including Cd, Sn, Sb, In, Zn, Mn, Te, Bi, or the like is dispersed in Ag can be used as an electric contact (hereinafter referred to as "contact") with a base metal in a current switch. In particular, the Ag-oxide contact material exhibits excellent contact characteristics in view of deposition- and wear-resistance and, therefore, is employed mainly in a medium load range.
With the marked progress of rationalization and automation in every industrial field, mechanical equipment tends to be large and complicated. The requirements for switches for governing control over such machinery, on the other hand, include being compact in size, large in capacity, and able to withstand frequent operation. Because of frequent switching operation of equipment, the switch contact dramatically heats up to the extent that the contact is locally fused by arc and electrically-induced heat. Then, when it is out of operation, the contact is cooled down to the room temperature. The contact is, therefore, subjected to repetitions of heating and cooling cycles.
Normally, the contact is joined to a base metal when used for a switch. The contact is joined metallurgically by brazing or resistance welding.
When the contact is formed by brazing, the base metal is softened since the base metal and the contact have to be heated at high temperatures. The thickness of the base metal also has to be increased. Using the brazing method, therefore, is undesirable for reducing the switch in size. Moreover, the brazing method is unfit for mass production of switches because the automated operation of joining contacts and base metals is difficult.
A resistance welding method is superior to the brazing method because with resistance welding the base metal is less affected by heat, and the operation can be automated. Current is passed across the joint between the contact material and the base metal, and causes the material to be joined instantaneously. The contact material joined to the base metal by resistance welding is subsequently compression-molded vertically into a round or square contact.
FIG. 34 shows a process of joining a contact material to a base metal by resistance welding. FIG. 35 shows a contact formed by die forging of the contact material thus joined by, resistance welding. In FIG. 34, a contact material 1 is prepared by cutting a circular wire and laid in place on a base metal 2. Current then flows between electrodes 4A and 4B with the contact material 1 and the base metal 2 held therebetween. Due to contact resistance, electrically-induced heat is generated in the joint between the contact material 1 and the base metal 2. Thus, the joint is fused so as to weld the contact material 1 to the base metal 2 within a range of weld metal zone 3. The contact material 1 joined to the base metal 2 by resistance welding is vertically compressed by means of a mold (not shown) into a disclike contact 5 shown in FIG. 35.
Despite the advantage over the brazing method in being more easily automated and highly productive, it is still difficult to join the whole surface of the contact to the base metal by resistance welding. As is obvious from FIG. 35, the weld metal zone 3 exists only in the central portion of the contact 5 subjected to die forging. Therefore, if a large current is repeatedly turned on and off through the contact joined by resistance welding incorporated in an electromagnetic contactor, the contact 5 peels off the base metal 2, as shown in FIG. 36.
In FIG. 36, numerals 5 and 6 designate a fixed and a moving contact, respectively. The contact 5 is heated by an arc 7 when the contacts 5 and 6 are separated and contact 5 is cooled after the arc 7 is extinguished. However, the surface of the contact contracts during the course of cooling and consequently the force resulting from the concentration of the heat at the center is applied to the outer periphery of the contact 5 in such a direction as to make the outer periphery thereof peel off. Once the peeling starts, transmission of heat to the base metal 2 diminishes and this causes the contact 5 to be increasingly heated and peeled off. Ultimately, the contact may undergo abnormal wear or drop off from the basemetal 2. The arc is often driven by a magnetic force in a fixed direction (e.g., in the direction of arrow P of FIG. 36) during the period between its generation and termination. In this case, contact peeling tends to be biased toward the terminal end of the movement of the arc.
One solution to the problem of increasing the contact area is to use a large welding current. If, however, the welding current is increased, the wear of the electrodes used to supply the current also increases. As a result, the electrodes will need frequent repairs and high productivity will deteriorate.
Contact materials of Ag-oxide, such as Ag-CdO and Ag-SnO.sub.2 are preferred materials. These Ag-oxide contact materials feature high arc-resistance and, therefore, high adaptability for use against a large current. Thus, the joint strength is much lower than that of non-oxide contact materials because of the presence of an interfacial oxide formed with the base metal by the Ag-oxide materials. However, Ag-oxide materials tend to readily allow contacts to peel off. The peeling may be reduced by providing a silver backing layer is provided for a contact chip to which a base metal is joined by brazing. However, this method is difficult and cannot be automated.